The PN junction diode.

A perfect diode is a device that only allows current to flow through it in one direction, (i.e. it has infinite resistance to current flow in one direction and zero resistance in the other direction). The diode has two terminals called a Cathode and an Anode . The circuit symbol for a diode is shown below. When an external voltage is applied, with the positive terminal connected to the anode and the negative terminal connected to the cathode, then current flows through the device. However, if we reverse the polarity, a perfect diode will completely block current flow.

Diode Symbol

The forward biased resistance of the PN junction and the leakage current that flows when reverse biased are both very small. In practice we can regard them as negligible. Therefore the PN junction has suitable unidirectional properties, to be used as a practical diode. A typical semiconductor diode is shown below. The silver line indicates the cathode terminal. Diodes like this are manufactured with a range of different current ratings, to meet the requirements of different circuits.

Semiconductor Diode

Peak Inverse Voltage.

Semiconductor breakdown has been described previously. This is something that could occur in a reverse biased PN junction, if the applied voltage is high enough. This is something we need to avoid if the PN junction is to be used as a diode! Diodes are used to rectify a.c. voltages, (i.e. to restrict a bi-directional current flow, to one direction only ). a.c. voltages cycle repeatedly through a range of values and it is important that the diode can withstand the peak voltage, that will occur when it is reverse biased, i.e. the Peak Inverse Voltage (P.I.V.). Diodes are manufactured with a range of P.I.V. s and it is important to ensure that the diodes P.I.V. rating, exceeds the peak voltage that will be applied to it.

Diode characteristic .

The operating characteristics of a diode can be illustrated with a graph of the current through the diode plotted against the applied voltage. This is called a diode characteristic.

The negative part of the voltage axis corresponds to when the diode is reverse biased and the positive part is when the diode is forward biased. (The negative part of the current axis shows the leakage current flowing through the diode when it is reverse biased).

Diode Characteristic

Light Emitting Diodes (L.E.D.).

When discussing electron hole pair generation, it was explained that energy has to be supplied to valence electrons, to enable them to jump up to the conduction band. At the junction of a forward biased diode, the reverse situation occurs. i.e. Electrons from the conduction band in the N type semiconductor, drop down into the valence band in the P type semiconductor and emit energy in the process. This energy is given out as a photon of light. A "standard" semiconductor diode is encapsulated in an opaque material, so any light emitted at the junction would not be seen. LEDs are designed to utilise this effect, as a basic lighting component. In order to achieve this, firstly, the doping materials are chosen to create a specific energy gap between the conduction and valence bands at the junction. This determines the energy of the photon that will be emitted and therefore the frequency of light that will be given out. Secondly, the encapsulation material is transparent and is shaped into a lens to focus the light that is given out. The diagram below shows a typical light emitting diode, (the flat section on the lens indicates the Cathode.)

LED (public domain image)

Zener Diode.

We have discussed previously that a diode can experience zener breakdown without permanent damage. Diodes can be manufactured to have a very specific reverse breakdown voltage. A Zener diode is designed to be reversed biased in a circuit and breakdown at a very specific voltage to act as a voltage reference device.

Diode protection.

Once the barrier voltage has been exceeded in a forward biased PN junction, the device becomes highly conductive. This is apparent from the diode characteristic, which shows a rapid increase in current for a small change in the applied voltage. This would tend to cause a large current through the diode that would quickly overheat and then destroy the device. For this reason a resistor with a suitable value must be placed in series with the diode, to ensure that the current does not exceed the current rating for the device. (This also applies to Zener diodes, which will conduct in reverse bias once the reference voltage has been exceeded)

Varactor (or Varicap) Diode.

We know that in a reverse biased PN junction the depletion region acts as an insulator between the two conductive regions. This is the basic structure for a capacitor. Therefore a reverse biased PN junction can act as a capacitor. Furthermore the width of the depletion region can be controlled with the reverse bias voltage. This will vary the capacitance of the device. Devices manufactured for use in this way are called varactor or varicap diodes.